Wind effects on Circumpolar Deep Water intrusions on the West Antarctic Peninsula continental shelf Mike Dinniman John Klinck Center for Coastal Physical Oceanography Old Dominion University Norfolk, VA, USA

Outline of Presentation Introduction Description of circulation model Timing of Circumpolar Deep Water (CDW) intrusions on the WAP “Downscaling” simulations: CDW transport and ice shelf basal melt Conclusions

Introduction Relatively warm CDW found at shelf break around most of Antarctica Changes in either the temperature or quantity of warm CDW entering the ice shelf cavities have been proposed as a reason for the increased volume loss of the WAIS Modifications in the wind forcing have been proposed as a mechanism for changes in the flux of CDW onto the continental shelf and to the cavities beneath the ice shelves

Antarctic Peninsula Model ROMS: 4 km horizontal resolution, 24 levels ROMS: 4 km horizontal resolution, 24 levels Ice shelves (mechanical and thermodynamic) Ice shelves (mechanical and thermodynamic) Dynamic sea ice Dynamic sea ice Bathymetry: ETOPO2v2 + WHOI SOGLOBEC region + Padman + BEDMAP + Maslanyj Bathymetry: ETOPO2v2 + WHOI SOGLOBEC region + Padman + BEDMAP + Maslanyj Daily (or better) wind forcing from a blend of QSCAT data and NCEP reanalyses or Antarctic Mesoscale Prediction System (AMPS) winds Daily (or better) wind forcing from a blend of QSCAT data and NCEP reanalyses or Antarctic Mesoscale Prediction System (AMPS) winds Experiments w/ dye representing CDW Experiments w/ dye representing CDW

Previous view of WAP intrusion frequency: 4- 6 events/year However, this was based on broad scale hydrographic surveys Detided, low-pass filtered time series of potential temperature at two moorings within Marguerite Trough => shows much higher intrusion frequency Moffat et al, 2009 Solid line: A2 – north side of MT Dashed line: A3 – south side of MT

Model histogram of the intrusion duration (50 dye unit- Sv. threshold) for the entire model run Model: 1.9 events/month (3.3 events/month w/ 40 dye unit- Sv. threshold) Typical duration: 1-4 d Observed histogram of the intrusion duration A2: 3.8 events/month Typical duration: 1-3 d Definition of intrusion is arbitrary, but model is much closer to mooring data than previous survey based picture of 4-6 intrusions per year

Duration of intrusions suggests wind forcing (weather band frequency: 2-8 days) No significant correlation between wind perpendicular to flux “gate”, but significant lagged correlation for wind parallel to cross-section: r = 0.55 (wind leads dye flux by 2-3 days) Red: Pseudo-stress (x 1/3) parallel to CDW-dye flux cross section Blue: CDW-dye flux anomaly (reversed sign)

Wind Change Simulations Positive Southern Annular Mode leads to stronger westerlies in this area Simulate regional effects of this with modified winds ACC transport in a regional model must be imposed: Simulate global effects by increasing the ACC transport Simulations run for 3 years that correspond to Reference case using current conditions (QSCAT/NCEP) Winds are simply scaled with both components multiplied by a constant factor ACC transport increased by sharpening the T and S gradients in the ACC fronts on the lateral open boundaries

Stronger winds and ACC provide more CDW to open shelf, but not necessarily to the cavity beneath GVI Ice Shelf Dye concentration for Margeurite Bay Dye concentration beneath GVI Ice Shelf

Melt rate beneath George VI Ice Shelf Stronger winds reduce melt rate beneath GVI (stronger ACC makes little difference) Why?

Dye concentration Temperature Base windsWind X 1.2 South GVI entrance: Feb. 2001

Conclusions Intrusions of CDW on to the shelf have a much quicker timescale then previously thought and the timescales seem to be related to wind events Intrusions of CDW on to the shelf have a much quicker timescale then previously thought and the timescales seem to be related to wind events Increases in winds and ACC transport lead to increases in the amount of CDW advected onto the continental shelf, but do not necessarily lead to increased CDW flux underneath the ice shelves or increased basal melt Increases in winds and ACC transport lead to increases in the amount of CDW advected onto the continental shelf, but do not necessarily lead to increased CDW flux underneath the ice shelves or increased basal melt

Acknowledgements BEDMAP data courtesy of the BEDMAP consortium Other bathymetry courtesy of Laurie Padman and Tom Bolmer AMPS winds courtesy of John Cassano Computer facilities and support provided by the Center for Coastal Physical Oceanography Financial support from the U.S. National Science Foundation (ANT ).